High-frequency sound-transmission system



` c. A. SPRAGUE HIGH FREQUENCY SOUND TRANSMISSION' SYSTEM Filed June s'.192x Tr-4f( 1414141414/ other purpose.

Patented Feb, 8, i927.

Application le Jue 2l,

This invention relates to acoustic signaling' systems and moreparticularly to a system of this kind in which acoustic waves of frequecies abovethe limits of audibility 5 are ployed to reinforce, in therear of an ditorium, the ordinaryacoustic waves' set up by the voice ofa speaker or other sounds produced at the'front of the auditorium forinstruction, entertainment or The invention, so vfar as known, is amarked departure r from' acoustic systems which Ahave been previouslyldevised, and several features employed are capable of broad applicationin acoustics and signaling.` In order to' clearly describe theinvention, however, its application to auditoriums will first bedescribed. This is the field in which the invention in its entirety willprobably indits most useful application.

` The need of' a system for enabling auditors in the rear portionsoftheatres and churches and other assembly places to hear distinctly thesounds emanating from the stage orfrostrum has become urgent.- Recentlyso-called loud speaking telephone systems haved been developed which inameasure lill this need, but all such'systems have one or more of thedefects V'mentioned below, all of which are substantially overcome inthe system according to the present invention. l

If the reinforcing horn or instrument'is near the speaker, thereinforcement isgreatest in the front of the room, where it should beleast. To those near the speaker the sounds appear unnaturally-'loud andthis ef- 'fect .is sometimes so great vas to be very annoying.

- lf the reinforcing horn or instrument is in the rear of the room, orif many such instruments are distributed throughout the room -those nearthese instruments may receive the impression that the speaker is attheir rear. The range ,of frequencies employed (the audible range) is a`diiiicult one in which to .operate because in transforming sound en-.ergy to electrical energy and vice versa, distortion may be introduced,for example, by the generation of audible harmonic waves,

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By employing carrier frequencies above the audible range during thesetransformations, as in the system of this invention, the harmoniesproduced are inaudible and havelno effect upon the ear of the auditor.

When a single reproducing horn is einployed, the distortion due toresonance within the horn is marked.` This effect is overcome by usinginaudible frequencies.

Refering now to the accompanying draw- 00 lings, Fig. 1 is adiagrammatic view of an auditorium vequipped with this invention; Fig.y2 isa diagram of the high frequency electrical -circuit' arrangementdisposed near the speaker; Aand Fig. 3 is a diagram ofthe 05 highfrequency electrical circuit arrangement located in the rear of theauditorium. The auditorium 1, provide-d with seats 2 and platfrm 3 sequiped with apparatus 4 placed near the spea er for transforming 70 aportion of the energy of `the speakers voice into high fre uency soundwaves for transmission towar the rearof the room.

-These waves are modulated in accordance Mechanical 15' with the speechfrequencies. means can be used for producing these high frequencysoundwaves, but apparatus well adapted for this purpose has not-yetbeenF developed' and it is accordingly preferred atthe resent time touse electrical appaso ratus o wellv'known design.

' Referring to Fig. 2, an oscillation generating system O of the audiontype employing atube 6 is adjusted to generatefalter- Dating currents ofa frequency above the audible range in the oscillation circuit 7, 8,

9, in well known manner, the frequency bev ing determined b the amountof inductance and capacity in t is circuit. rlhe instrument '9 is adevice for setting up sound waves of 90 enerated current and vmayconsist of what is commonly called a condenser receiver,- many forms ofwhich have previously been devised. A parabolicsound 'reflector 10,preferably thick-walled 95 rojects the sound waves ina beam bof sustantiallyparallel rays to the concave sound reflector 11 (Fig. l). Thelatter reects and focuses the sound beam upon the diaphragm of the soundreceiving apparatus1 20 to be later described.'

A modulating. system 12 of the audion type receives hlgh frequencyenergy from generator 0 ad is simultaneously supplied with voicefrequency energy from a telephone transmitting circuit 13. The telephonetransmitter is located where ,a por.- tion of the speakers voicewillreach it.

The audion' 14 ampliiie's the high frequency electrical waves impresse-dupon, it

and the amplif ing power of the tube 4is controlled by tie voltageimpressed from the transmitting circuit 13. 'The result is to Aproducehigh frequency variations in the output current the amplitudes of whichaccord with the speech waves impressed on the input circuit. This is awell known form of modulating system and is shown merely by way ofexample. It was invented by E. H. Colpitts and J. H. Van der Bijl andfor a more detailed description the reader is referred to U. S. Patent(Reissue) No. 14,380 to Colpitts and U. S. Patent No. 1,350,752 to Vander,Bijl.

The high frequency modulated waves thus produced are impressed throughrtrans' former 15 upon an electric Wave filter 16 of the type disclosedin U. S. patent to Campbell, No. 1,227,113, granted May 22, 1917, thefilter being designed, in 'the manner described in the patent, to passall frequencies above certain value and to suppress all frequenciesbelow this value. It is 'now well known that a high frequency modulatedwave is in effect a plurality of wavesv of different frequencies whichmay be separated from each other by electric filters. If C representsthe frequency of wavesv generated by the generator 0, and S representsthe instantaneous frequency of the composite speech wave, the modulatedhigh frequency wave may be considered as made up of three waves havingthe frequencies C-I-S, C-S and C. The component of C frequency may belooked upon as the portion of the original wave which has not beenmodulated, there always being such a component unless special means areused to obtain complete modulation. Since S isa constantly changingfrequency, G+S and C-S are likewise continually varying. For example, ifC is 30,000 and S varies from 100 to 5000, G+S will vary from 30,100 to.-35,000 and C-S will vary from 29,900Y to,

25,000; .For a detailed disclosure of a mathematical analysis of amodulated electric wave see British patent 102,503 for an invention ofJohn R. Carson relating to .modulation The filter 16 is designedl totransmit only the component C-l-S4 or the component C-S and tosuppressthe others.

Therefore, 'although the transmitted fre quency is constantly changing,it never falls 'to awalue which is audible.

This band of changing frequencies is imvice 18 similar in function to 9pressed through transformer 17 u on a defor transforming the electricIwaves into sound Waves of like frequencies. A meaphone 19 transmitsthese sound waves the form of a beam a toward the rear ofthe auditorium.

A cross-section of this bea-m, due to the shape of the megaphone, isgreater in its horizontal than in its vertical dimension.

If the beam a were to be employed alone, no effect would be producedupon the auditors; but by simultaneously producing a sound of frequency.Cp in the region infwhich an amplifying effect is desired, thedetecting property of the ear is brought into play an the voice of thespeakerbecomes audiblel even though his voice cannot directly be heard.This is due to the phenomenon of acoustic beats. Whenever two soundwaves of different frequencies are simul-l taneously impressed upon avibratable body which distorts the waves, their energy is absorbed andreappears in the form of new waves, not only ofthe frequencies of theimpressed waves, but a third wave which has a frequency equal trthe sumofthe im pressed frequency waves, a fourth having a frequency equal tothe difference between those of the impressed Waves and others offrequencies harmonic to those of the im- 95 pressed waves, that is,integral multiples of these frequencies. The amplitudes ofV the sum anddifference frequencies will in general be much greater than those of thehigher harmonics and 'will be roportionate'100 to the product of theamplitu es of the impressed waves. Therefore this amplitude can be madegreat by increasing the am litude of one of the impressed waves. hevibratable elements of the ear have the property of distorting highfrequency waves impressed upon them to produce such a series of waves asthat just described. Thus, when thefrequencies G and C-l-S, for example,are impressed upon the ear there is produced a wave having a frequencyequal to the difference lof these Waves, i. e.

In other words speech is reproduced. -At the same time other waveshaving frequencies of 2C-l-2S, 2C, 2C{ S etc. are-produced, 115

' but each of these is above the limit of audi- Ibility and produces noeffect.

'If a wave of frequency C-S rather vthan C-i-S is impressed upon the earsimultaneously with a wave of frequency C, the result, so .farasvlao theproduction of audible frequencies 1s concerned, is the same, i. e., thedifference frequency is S, and the sum frequency and harmonicfrequencies are inaudible.

The purpose of the apparatusshown in 125' Fig. 3,is to set up in therear of -the auditorium a wave of the frequency C andof constantamplitude, which reaches the auditors at the same time as the sound offre'- quency C-l-S (or C -S) from the front of Lacasse the room. A soundgenerator, either mechanical or electrical, capable of producingfrequency C, could be operated at `the, rear of the auditoriumindependently of the high frequency sound produced near the'speaker, ifit were possible to maintainl the frequency C constant, or exactly thesame as the carrier frequency pro ced at the front-of the room. It is soditiicult to do this, however, that it is lpreferable to providesynchronizing means between the generators at the' front and the rear ofthe room. Any known means for producin the desired synchronism may beemploye The system shown in Fig. 1 is being here described by Way ofexample. Strictly speaking, this is an amplifying rather` than asynchronous generating system in that the beam b of sound Waves istransformed into electrical waves of corresponding frequency, which arethen amplified and reconverted into sound waves. -The lbeam b isreflected and focused by the concave relector 11 upon the diaphragm oftelephone transmitter 20, which Ipreferably has a natural frequencyequal to or greater than that of the impressed waves but which'may beprovided with damping means. Any other form of device for transformingsound waves into electrical Wavesmay be substituted for the transmitter20. The .Waves thus set up are amplified by the thermionic vacuum tubeamplifier A and are reconverted into sound Waves by means of theconverter 22 in circuit 21, 22, 23. The converter 22 may be of any knowntype but is shown as a so-called condenser-receiver. As the potentialupon the condenser elements is varied in accordance with the Wavesflowing in the circuit 21, 22, 23, they are vibrated and thus set up.sound Waves in the air. The circuit 21,' 22, 23 may be tuned to thefrequency transmitted as may also the circuits of jsound transmitter 18and sound receiverI 20. 'A damping resistance 23 may be inserted incircuit 21, 22, 23, as shown, and may be of.

such valve as to give -the circuit any damping characteristic desired.`A similar resistance may be used in ,the other circuits vjustmentioned. Thesource 21 of -directpoten# tial provides means forpolarizing the condenser-receiver 22 and may be omitted if desired. Itis to be understood that Whenever.A

a condenser-transforming device is employed it may if desired belpolarized in well-known manner. `The sound waves thus set up aretransmitted as a. beam c toward the front of the room, a crossfsectionof this being, as in the case of beam a, greater in its horizontal .thanin its vertical dimension. By means of the amplifier A any desireddegree ofamplif ication may be obtained, a plurality of am'- plifiersbeing employed if necessary. The

wave amplitude of the beam c is preferably muchigreater than thatoffbeam a; yThe operation of thersystem should be apparent from theabove description-` The sound beams a and o are simultaneously impressedupon the ears of the auditors throughout the room excepJ in a zone nearthe speaker, in which his oice can'readily be heard. `Assuming, forexample, that the beam a consists of -a band' of frequencies C-i-S (theeffect being the same, however, whether the band is one of G+S or C-Sfrequencies)l there is produced in the ears of each auditor who receivesthese .waves a series of waves of frequency or frequencies S in themanner described above, i. e., the ear functions as a detector to renderperceptible the low frequency modulating com- "1 ponentof the impressedwaves in a manner analogous to the operation of a so-called detector ofelectrical rwaves at a receiving radio telephone station.. y

The auditor therefore, if he is Within the range of the speakers voice,hears the speaker both directly and indirectl effects are additive. Thee ect which is pro'- duced indirectly-is, however,` greatest inmagnitude in the rear of the rgom and is progressively less from theretoward the' front and may be made of such value as to render the sum ofthe indirect and'direct effects approximately Vuniform throughout theroom." .The reason why the eect 1s greatest at the rear of the room anddecreases progressively from there toward the front of the room is that,as explained above, the amplitude of the difference frequency'producedwhen two waves simultaneously impinge upon a.

element is proportional to the detecting product of the amplitudes ofthe impressed waves. Therefore by making the initial a'mplitude of thebeam c very much g greater than that of the beam a the product of theseamplitudes will be greatest where the amplitude of the beam o isgreatest, that is, in the rear of the room.

Since the high frequency sound wave which is proceeding from thevicinity of the speaker is varying 1n accordance with speech, while thewave from the rear of the room is of constant amplitude and freqtlency,the effect upon the ear is the same as though the entire energy receivedpioceeded from-the vicinity of the speaker. This is considered' animportant feature of this invention, since, by reason of it, the sourceor sources of plified waves ma be placed in the rear of the room ordistributed throughout the room Without producing the incongruous effectof seeing the speaker in one direction and hea ing him in, another.

` As just suggested, as many sound sources of frequency C may be used asnecessary to give approximately uniform sound intensityA throughout theroom. It lwould, o'f course,-

and these two f lof beam a may be widened by a properly shaped horn ormegaphone, or several sound transmitters such as 18A may be employedsimultaneously and energized from the sameV '2 thel reflector 11 and thetelephone transmitter 20. #In this way the two high frequency waves maybe made to reach the auditors in any desired phase relation and sincethe pheno enon is one of beats the phasel of thegegt Wave with. respectto that of a wave e same frequency from the speaker d epends upon thephase relation between the sound beams a and c. The wave length of thewave having the frequency C isa few inchesor at most a few feet so thatthe phase relation between the two high fre uency waves of beams a and cmay be varie from zero to 180 degrees by a change of a few inches'or atmost a few) feet in the position of the producing or reflectingapparatus.

In case the installation is to be permanent and indoors, it isordinarily preferred io omit the condenser 7 'the `sound producer 9, thereflectors 10 and 11 and the sound receiver 20 with its. battery, and toconnect the amplifier A with the generator 0 by means of an electricallyconductive circuit extending along the Hoor lor wall of the room. Such acircuit may, of coursefbe employed also when the installation isoutof-doors if desired.

While the invention has been described.

as of particular utility in a sound amplifying system, it is alsoapplicable to relatively long distance aerial or .submarine secretsignaling.' 'I he transmitting operator thenv speaks directly intothetransmitter 13 and l the ear of theY receiving operator is placed .ofthe sound from small.

near the sound producer 22. The amplitude 22 may then be relatively Anacoustic detecting instrument may be employed in the place of the ear,or as an auxiliary thereto.

The terms sound and acoustic are herein employed in their broader senseto denote material waves of frequencies lying outside the audible rangeas well y'as those within that range.

What is claimed is:4 1. An acoustic wave system comprising means forproducing-,low frequency sound waves, a. source of high frequency Waves,

means for modifying said high4 frequency waves in accordance with` saidlow frequency waves, means for suppressing all of the comonents of themodied wave except one side band of frequencies, means for directivelytransmitting to a receiving point an acoustic wave comprising said sideband frequencies, a source of acoustic waves of the'same fre- 'quency asthe said first-mentioned high frequency source, a Wave distorting devicefor producing waves of sum and difference frequenciesgfrom impressedwaves, and means for impressing said acoustic wave having sideband`\frequencies and a wave from said last-mentioned-high frequencysource simultaneously upon said Wave distorting device.

2. An adoustic s stem comprising means for producing audi le sound.waves, a source of high fre uency waves of frequency above the audiblelimit, means for modifying said high frequency Waves in accordance withsaid audible waves, means for suppressing all. of the co l ponents ofthe modulated wave except one side band of frequencies, means acousticWave comprising said side band frequencies, a source of acoustic wavesof the same frequency as the said first-mentioned high frequency source,and-means for im-V pressing said acustic wave having side bandfrequencies `and a wave from said last-mentioned high frequency sourcesimultaneously upon the ear of an auditor.

3. A- sound wave reinforcing s stem comprising means for producing auible sound waves to be reinforced, a source of high frequency waves offrequency above the audible limit, means for modifying said inaudiblewave in accordance with said audible waves, means for suppressing allthe com onents of the modulated wave except one si e band offrequencies, means for transmitting to a receiving point an acousticwave com rising said side band frequencies, a source o acous-f tic wavesof the same frequency as said firstmentioned high frequency source, andmeans for impressing said'acoustic lwave having 'side band frequenciesand a wave from said last-mentioned high frequency source simultaneouslyupon the ear..of an auditor. p

In a Wave transmissionv system, the method of reinforcing audiblefrequency waves at a desired point in space which comprises combiningtwo inaudible wave's with each other and with said 'audible frequency tlatediin accordance with signals, transmitting said modulated vwavesthrough space, and directively transmittino' and reecting saidunmodulated waves whereby they are caused to combine at a` desired pointwith said modulated waves.

7. A signalx transmitting system which comprises meansforproducingun'modulated l inaudible waves', means for producinginauwhich comprises generating and transmitting an inaudible acousticwave having its frequency modied -in accordance with a signal separatelygenerating an inaudible wave, and making the signal'audible byimpressing upon the ear simultaneously 'therewith said second inaudiblewave.

10. The method of acoustic wave signal-` ing which comprises impressingsimultaneously upon`tlie ear of an auditor an acoustic wave of constantfrequency and -a separately generated and transmitted wave of varyingfrequency. l

11. The method of acoustic wave signaling which comprises generating andtrans-'I mittinga high frequency acoustlc Wave modulated in accordancewith the signal, separately generating an unmodulated high frequencywave and simultaneously impressing said Waves upon theear of an auditor.

12. The 'method of acoustic signal transmission from substantially theposition of an audible frequency source to an audience,

Vwhich comprises transmitting from substantiallythe position ofl saidsource, audible fre uency waves as modulations of an inaudible frequencyacoustic Wave, and directively transmitting acoustic waves of saidinaudible frequency from a pointother than that of said source.

13. The method of ,acoustic signal transmission fromv substantially theposition of an audible frequency source to an audience,

' which compris transmitting ,audible frequency acoustic waves from saidsource,

'transmitting lfrom substantially the position of said source aninaudible frequency acoustic wavemodnlated by waves of said audiblefrequency and directively transmitting acoustic waves of said inaudible`frequency from a point other than that of said source.

14. The method of acoustic signal transmission from a desired point toan audience which comprises transmitting audible 'frequency acousticWaves from said point to the audience, transmitting from said pointwaves of said audiblefrequency as modulations of an inaudible vfrequencyacoustic wave, and transmitting acoustic waves of said inaudiblefrequency and of greater amplitude than that of said modulated wavesfrom a point at the op osite side of the audience from said desirepoint.

15. The method of acoustic signaling froma desired point to an audiencewhich comprises transmitting an' audible frequency acoustic Wave fromsaid point to the audience, modulating an inaudible frequency acousticwave in accordance with said audible .frequency Wave, transmitting themodulated components of said wave, to the audience, se aratelydirectively transmitting the unmodu ated componentto a point on theopposite sider of-said audience from said point, generating by means ofsaid unmod` ulated component at said second point a Wave of the samefrequency but of substantially greater amplitude, and transmitting saidwave-to the audience.

16. The method of 0acoustic wave transmission which comprises modulatingan acoustic inaudible frequency carrier wave in accordance with a lowfrequency wave in such manner that the resultant modulated wave has acomponent equal` in amplitude to the product of the carrier and `lowfrequency' waves, transmitting said modulated wave to the ear of anauditor and impressin upon the ear simultaneously with the mo ulatedwave a. second inaudible wave of the-same fr uency as said acousticinaudible Wave.

el? A sound operated amplifying appa-l ratus for an auditoriumcomprising sound receiving means located ata point Where sound from asource has become attenuated, and electrical means for reinforcing soundwaves controlled bysaid sound responsive means. J l

18.` The method of producing the sensation of audible speech upon theear of an observer Which comprises impressing upon the natural physicalmedium in operative proxl'mity to the ear a compressional wave offrequency above theusual audible range 1nher'ently containing theelements of speech and another Wave of frequency! above the usualaudible rangev y 19. A system comprising separate elements forproducingcompressional waves-of frequency above the usual audible range,means for causing one of said waves to vary 1n accordance with speech,and means for im-V pressing said Waves upon the ear of an observerwithpdesired relative intensities.

20. A system for producing in one and the same physical medium at leasta part of the .frequency components of a compressional wave varyingnaccordance with speech and an unmodulaed compressional component of saidwave, and means for independently regulating -the intensity'of. saidunmodulated 5 component. Y

21. A system comprising separate wave producers and associated elementsfor pro- 'ducing in th'e same body of meium a compressional wave speechside band', and a, eompressional wave corresponding to the 10unmodulated component of said side band. AIn witness whereof, I hereuntosubscribe my name this 2nd ay of June, A. D. 1921.

CLARENCE A. SPRAGrUE.v

